Gapless Play & How to Test for It  (December 2017)

 

Gapless playback, mentioned on the Our Services — and More page, is what happens when there is no perceivable silence between contiguous tracks.  This of course is the desired state when one song immediately transitions into another, or when adjacent songs are crossfaded.  CD players pretty much all support gapless play, and most CD burning programs provide an option for gapless play (sometimes invoked by not selecting the option to insert one or two seconds of silence between tracks).

 

Gapless playback is not a universal feature of audio player software, including MP3 players.  While many support gapless play, others do not, and therein lies a problem.  If a continuous recording is split into tracks and converted to MP3 files, and those files are then played on a player that does not support gapless play, you’ll hear about a half second of silence between tracks.  This is disruptive to the listening experience, if not outright irritating.

 

Part of the problem stems from ID3v2 tags, which appear at the start of the file before the audio track begins (see Labeling MP3 Tracks:  ID3 Tags).  The player needs to wade through the ID3 tag, and if it doesn’t look far enough ahead to begin buffering the next track’s audio data while the current track is still playing, you’ll hear a gap.  (To give you an idea of the processing needed for gapless playback, a “fully loaded” ID3v2 tag can be as large as 256 MB, while a three-minute MP3 file is about 3.5 MB.)

 

Another cause is that converting the original, lossless audio file (e.g., WAV) into a compressed, lossy format like MP3 adds a small amount of silence to the beginning and end of the track.  Moreover, there is nothing in the MP3 metadata that indicates how long the added silence runs, information an audio player might have used to skip over the inserted silence.  Consequently, while transitioned or crossfaded WAV files play with no perceivable gap, a very brief gap will be evident with converted MP3 files, even on MP3 players that support gapless play.  The only way around this anomaly is to manually edit the MP3 files after their creation, deleting the silence that the conversion process added.

 

When a customer wants MP3 files, I will ask if the customer’s players support gapless play.  If they do, all proceeds normally.  If not, I will suggest combining transitioned or crossfaded tracks into a single MP3 file.  The customer gets to choose which route to take.  (I will work with the customer to create an appropriate track name for a combined MP3 file, something better than “Side 2” but briefer than several concatenated track titles.)

 

How would customers know if their MP3 players support gapless play?  They probably wouldn’t, unless they had previously encountered gaps between transitioned or crossfaded tracks.  So, I’ve concocted a test.

 

I created two MP3 files consisting of five seconds of a 440 Hz tone; each file is less than 200 KB.  In Part A, the tone fades in to its highest volume, and in Part B the tone starts at its highest volume and fades out (See Figure 1).  Email me at paul@playitagainpaul.com, and I’ll respond to your note with the two files attached.

 

• Download both files and load them onto your MP3 player.

• Set them up to play Part A immediately followed by Part B.

• Play the two tracks.

 

If your device supports gapless play, you should hear the tone fading in then fading out with no silence inserted in the middle.  If your device does not support gapless play, you’ll hear the tone fade in, a brief period of silence, and the reappearance of the tone that will end in a fade-out.  (If you hear the tone starting off at its highest volume, fading out, and fading back in to its highest volume, you’re playing them in the wrong order.  Change the order and try again.)